![]() Gabbro is also found with other plutonic rocks in batholiths when bodies of rising magma are low in silica. That makes gabbro a key sign of an ophiolite, a large body of oceanic crust that ends up on land. Gabbro makes up most of the deep part of the oceanic crust, where melts of basaltic composition cool very slowly to create large mineral grains. You can get away with calling almost any dark, coarse-grained igneous rock gabbro, but true gabbro is a narrowly defined subset of dark plutonic rocks. Gabbro is named after a town in Italy's Tuscany region. The other dark minerals may include amphibole, pyroxene, and sometimes biotite, olivine, magnetite, ilmenite, and apatite. Also, gabbro has no alkali feldspar, only plagioclase feldspar with a high calcium content. Unlike granite, gabbro is low in silica and has no quartz. Gabbro is a dark-colored igneous rock that is considered to be the plutonic equivalent of basalt. State of New South Wales Department of Education and Training
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